This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Entry to sporulation in Bacilli is governed by a histidine kinase phosphorelay, a variation on the predominant signal transduction mechanism in prokaryotes. Sda directly inhibits sporulation histidine kinases in response to DNA damage and replication defects. We determined a 2.0 [unreadable]-resolution X-ray crystal structure of the intact cytoplasmic catalytic core (comprising the Dimerization and Histidine-phosphotransfer, or DHp, domain, connected to the ATP-binding Catalytic, or CA, domain) of the Geobacillus stearothermophilus (Gst) sporulation kinase KinB complexed with Sda. Structural and biochemical analyses reveal that Sda binds to the base of the DHp domain and prevents molecular transactions with the DHp domain to which it is bound by acting as a simple molecular barricade. Sda acts to sterically block communication between the CA and DHp domains required for autophosphorylation, as well as to sterically block communication between the response regulator Spo0F and DHp domain required for phosphotransfer and phosphatase activities.